Electronic weigh scales generating digital outputs are known in the art. It is typical in such an electronic weigh scale to employ a load cell which generates an analog weight signal proportional to the weight of the object to be weighed. This analog weight signal is amplified via an analog amplifier and then applied to an analog to digital converter. The digital weight signal proportional to the analog weight signal is then applied to a computing device which may be in the form of a microprocessor. This computing device then converts the raw digital number into a weight which may be indicated in metric or English form.
The use of a computer operating upon the digital weight enables the electronic weigh scale to be advantageously employed in a number of ways. In particular, it is relatively simple to indicate a tare weight and subtract that from the gross weight in order to obtain the net weight. For example, the container alone can be placed upon the platform of the electronic weigh scale and the tare weight measured and stored. The scale can then be zeroed by subtracting the tare weight from the total weight. Thus, after the addition of the contents of the container, the resulting weight is the net weight rather than the gross weight. Such a scale can also be employed as a counting device of objects of identical weight. This is achieved by weighing one or more of the objects to be counted and computing the average weight. Then a quantity of the objects to be counted is introduced onto the weighing platform. The total weight is divided by the previously computed average weight in order to obtain the count of the number of objects on the weigh platform.
In addition to the usefulness of being able to digitally manipulate the weight or weights of objects in the computing device, it would also be advantageous to provide a plurality of weight ranges in such an electronic weigh scale. A multiple range function would enable the same apparatus to provide weights having differing full scale ranges based upon the differing sensitivity. Heretofore, such multiple range electronic weigh apparatuses have employed either multiple load cells or analog range switching.
A multiple range electronic weigh scale employing multiple load cells typically provides outputs of differing sensitivity to the same weight upon the weigh platform. The multiple load cells of such a multiple range electronic weigh scale in accordance with this teaching of the prior art are generally disposed in tandem so that the most sensitive load cell is first actuated followed by successive actuation of load cells having decreasing sensitivity. An example of such an electronic weigh scale is U.S. Pat. No. 4,690,230 entitled "Multi-Range Load Cell Weighing Instrument," issued to Uchimura et al. on Sept. 1, 1987. Such multiple range electronic weigh scales typically involve complex mechanical configurations in order to enable the cooperation of the load cells of differing sensitivity. Accordingly, it would be advantageous in the art to provide a multiple range electronic weigh apparatus which provides the multiple range capability primarily in the electronics of the apparatus. Because the electronics can typically be configured in a much wider range of flexible configurations than the mechanical parts of such an electronic weigh scale, it is considered advantageous to employ a pre-existing load cell with new electronic circuits in order to provide a desired multiple range capability.
Also known in the prior art is a multiple range electronic weigh scale employing analog switching between weight ranges. U.S. Pat. No. 4,696,359 entitled "Electronic Weighing Apparatus," issued to Glibbery on Sept. 29, 1987 describes such a system. The analog signal from a single load cell, after preliminary signal conditioning and amplification, is applied to a voltage comparator. This voltage comparator compares this analog signal to a preset voltage corresponding to the weight range boundary. The voltage comparator output controls the gain of an amplifier which receives the analog signal as an input and drives an analog to digital converter. When the analog signal is below the preset voltage a higher gain is selected. When the analog signal is above the preset voltage a lower gain is selected. This technique lacks the flexibility provided by the use of a digital computing device to manipulate the weight or weights. In particular the provision of more than two ranges according to the technique of U.S. Pat. No. 4,696,359 would require additional voltage comparator circuits.